The recent development of sensitive assays which measure thrombopoietin levels in serum has made analysis of thrombopoeitin possible. The objective of this proposal is to define the normal range for concentrations of circulating thrombopoietin (TPO) in healthy term and preterm infants without thrombocytopenia and to compare them with TPO levels in thrombocytopenic infants. Status: Subject enrollment and data analysis completed.